The powdered metal is firstly compressed in a die to form a "green" preform or compact having the general shape of the die. The compact is then sintered at an elevated temperature to fuse the individual metal particles together to form a unitary sintered metal part having a useful strength and yet still retaining the general shape of the die in which the compact was made.
Thereafter the shaped component is then machined to its final form for example by drilling, tapping and turning.
Metal powders utilized in such processes are generally pure metals, or alloys or blends of these, and sintering will yield a part or component having between 60% and 95% of its theoretical density. If a particularly high density is required, then a process such as a hot isostatic pressing will be utilized instead of sintering.
Brass alloys used in such processes are comprised of approximately 10% to 30% of zinc and 70% to 90% of copper.
Solid lubricants can also be included in the components and these are typically waxes, metallic/non-metallic stearates, graphite, lead alloy, molybdenum disulfide and tungsten disulfide.
For many metallurgical purposes, however, the resulting sintered product has to be capable of being machined, that is to say, it must be capable of being machined without either tearing the surface being machined to leave a rough surface or without unduly blunting or binding with the tools concerned.
It has, hitherto, been common practice for a proportion of lead in an amount up to 10% to be included by way of alloying within the material and to aid and improve the machinability of the resulting product. Lead is, however, a toxic substance and the use of lead in the production of alloys is surrounded by legislation and expensive control procedures. Furthermore, the lead phase in copper lead alloys can be affected by corrosive attacks with hot organic or mineral oil. For example when temperature of such an alloy rises, it has been known that the oil can break down to form peroxides and organic gases which effect a degree of leaching on the lead phase within the alloy. If this leaching progresses to any appreciable extent, the component, if it is a bearing or structural component, may eventually malfunction or fail.
There is, therefore, considerable advantage in reducing, or if possible, eliminating the contents of lead within powder metallurgy compositions. Various proposals have been put forward for doing this. The considerable proportions of lead incorporated in powder metallurgy materials in the past has resulted in ease of machinability and durability of the resulting product component. Replacement of part of the lead by bismuth has been proposed in our co-pending International Application published under No. WO91/14012. This results in successful replacement of part of the lead without a significant reduction in the machineability. It is, however, accompanied by some reduction of transverse strength of the material. For many purposes this reduction in transverse strength is not a significant problem.